The invention uses various materials which are electrically either conductive, insulating or semiconducting, although the completed semiconductor circuit device itself is usually referred to as a "semiconductor". One of the materials used is silicon, which is used as either single crystal silicon or as polycrystalline silicon material, referred to as polysilicon or "poly" in this disclosure.
In the operation of certain semiconductor circuit devices, it is necessary to draw up a node of the sense amp to a potential above V.sub.CC. In the conception of the present invention, these nodes occur on iso (isolation) devices on an array of a memory device and on word lines. Memory devices which use such iso devices on an array include DRAMs (dynamic random access memories). A typical arrangement of DRAM memory cells with a sense amp is shown in FIG. 1. Other types of memory devices, such as static RAMs and video RAMs also may have similar circuit arrangements. An isolation device (iso device) is present in order to isolate a digit load from the sense amp, so that the sense amp can amplify the signal faster than if the digits were directly attached to the sense amp.
Gating an iso device with a higher potential speed read time reduces the required size of the iso device. These results are obtained because an increase in potential increases V.sub.GS.
In a DRAM, the iso device is used with either multiplexed or non-multiplexed sense amps. In the case of multiplexed sense amps, reducing the size of the iso device allows configuring the circuit layout with the iso-devices "on pitch" (two pitch) rather than in a four pitch pattern. This simplifies layout design because the two pitch layout provides a configuration in which, for each sense amp, both iso devices are individually aligned with that sense amp. With four pitch layout patterns, more than one sense amp must be balanced as a unit.
The decrease in device width is obtained because increasing potential to gate gives the device a greater effective electrical transistor width.
In the prior art, bootstrapping had been used in order to charge nodes of a circuit to an increased potential. A typical bootstrap circuit is shown in FIG. 2. The bootstrap circuit provides an increased voltage level in response to a particular sequence, such as the receipt of a timing signal. The charge pump, on the other hand, provides a continuous output and an increased potential. The continuous high-potential output means that timing of the high-potential supply is not critical. This is particularly important when a high-potential node is used for the word line of a DRAM memory device, since the time required to select and address the word line is critical to the access speed of the DRAM.
Because the boot strap circuit is timed, individual boot strap circuits must be provided for each of several nodes, each of which receives current at the elevated potentials at different times. The charge pump, with its continuous output, can be used for supplying current to each of these nodes.
Boot strap circuits were also located at the location of the device controlled by the boot strap circuit. This was because individual boot strap circuits were dedicated to particular driven circuits in which different sequences were timed. Because the boot strap circuits were dedicated to particular driven circuits and because the boot strap circuits were timed to coincide with operation of the particular driven circuit, the amount of circuit area utilized by all of the circuitry was increased. This increase could occur despite the possibility of having smaller individual transistors in the driven circuits.
The ability to write a continuous voltage supply is particularly important in the case of word lines on DRAM devices. This is because, in DRAMs, word line addressing is particularly critical with respect to the speed of the part. Therefore, while providing a charge pump rather than a boot-strap circuit, the timing of the elevated potential output is not a condition precedent to word line access, simply because the charge pump output is not timed.
Prior art charge pumps consisted of an oscillator and capacitor. In order to prevent latchup, a clamp circuit was used in order to control current from the oscillator to one side of the capacitor. The use of an oscillator and capacitor with a single clamp circuit provided a relatively constant elevated potential, but was somewhat inefficient when compared to a boot strap circuit. It would be desirable to provide a charge pump circuit which has the relative efficiencies of a boot strap circuit but yet provides a continuous output such as is associated with a charge pump.
Prior art charge pumps use an oscillator and capacitor, along with a clamp circuit. The oscillator provides current at a supply potential to one side of the capacitor and the clamp circuit is used to charge the other side. The current supplied to the capacitor by the oscillator generates an increased potential at an output node of the circuit. This prior art circuit is shown in FIG. 3.
It is desirable to design an auxilliary circuit, so that in the event that the auxiliary circuit does not function as anticipated, the auxillary circuit can be bypassed. Specifically, during the design of an integrated circuit such as a DRAM, it is not known whether the electrical characteristics of the charge pump will exceed the limits of the circuit which receives the current from the charge pump. If the limits are exceeded, it is desirable to be able to make a small modification in the masks and thereby bypass the charge pump without sacrificing the remainder of the circuit design.
It is also likely that different product applications for integrated circuit parts may have different requirements of speed and supply voltage. If the auxilliary circuit provides a desireable function for one product application, but would make the part unsuitable for another product application, it would be desireable to be able to selectively bypass the auxilliary circuit. This would enable a single integrated circuit layout design to be used for both parts.
Ideally, an auxillary circuit should automatically respond to circuit conditions which makes the auxillary circuit unsuitable for its application. For example, a voltage boosting circuit would ideally attenuate its increased potential output or bypass itself as external system voltage becomes sufficiently high to make the use of the boosting circuit undesirable.